This year we have continued our studies of the molecular basis of drug- induced allergic hepatitis and have begun investigating those factors that may control the incidence of this category of drug toxicity. One possible explanation for the resistance of anesthesiologists to anesthetic-induced allergic hepatitis is that potentially damaging T cell-mediated immune reactions in the liver may be down-regulated through the cleavage of transmembrane-bound FasL (tmFasL) and subsequent release of soluble FasL (sFasL) from these cells into the serum. sFasL has been suggested to down-regulate the apoptotic activity of T cells by blocking the interaction of tmFasL with Fas expressing cells, such as hepatocytes. To test this idea, we have determined whether anesthesiologists have elevated levels of sFasL in their sera. Indeed, we found that serum sFasL levels of anesthesiologists were significantly higher than those of control and anesthetic hepatitis patients. In another study we investigated the potential role of resident T cells in the liver in the development of drug-induced allergic hepatitis. It was found that hepatotoxic drugs caused a dose- dependent decrease in CD3+ T cells of the liver 48 hours after they were administered to mice. Further experiments showed that CD8+ lymphocytes expressing gamma/delta receptors, but not other T cell subsets were predominantly depleted from the liver. Since T cells bearing gamma/delta receptors have been associated with autoimmune liver diseases such as autoimmune hepatitis, primary biliary cirrhosis, and primary sclerosing cholangitis, our findings suggest that these cells may be normally depleted from the liver when hepatocytes are damaged. This may suppress subsequent immunopathological reactions induced by autoantigens and/or neoantigens that are released from damaged hepatocytes. We have also investigated the molecular basis of hemolytic anemia caused by diclofenac, a widely used nonsteroidal anti- inflammatory drug. We report here that acute hemolysis develops in 20- 45% of mice following oral administration of diclofenac. Mice developed anemia as measured by a reduction in red blood cell and reticulocyte counts. Hemoglobin was found in the serum and urine of the affected mice. Toxicity also occurred in SCID mice suggesting that the mechanism is not immune based. Immunoblot analysis of erythrocyte membrane proteins with diclofenac-specific antisera revealed that diclofenac formed protein adducts of 108, 56, and 50 kDa. The 108 kDa adduct appeared to correspond to band 3, the RBC anion exchanger, because it reacted with antisera against this protein. These findings suggest that diclofenac-mediated hemolytic anemia may be due to the covalent alteration of one or more erythrocyte membrane proteins by a reactive metabolite of diclofenac. - Inhalation anesthetics, halothane, drug- induced hepatitis, immunopathology, diclofenac, hemolytic anemia, protein adducts